The present invention relates to thermal recording heads, and more particularly to a thermal recording head which is well suited to a video printer for obtaining the hard copy of a picture.
Heretofore, as a method of forming a full-colored picture print, there has been employed a thermal transfer method wherein a sheet of inks in the three colors of yellow, magenta and cyan (for example, a sublimable dye ink sheet) is used, and the inks are transferred onto printing paper by energizing a thermal recording head in which heating elements are disposed in one array. For the purpose of obtaining a picture print of high picture quality, in case of the A4-size by way of example, the number of the heating elements of the thermal recording head is usually selected at 1000 dots or above, and an electric power of about 100W is consumed on the average. As shown in FIG. 3, the prior-art thermal recording head is so constructed that the heating elements 25 are formed on an alumina-ceramic base plate 24, and that a radiating plate 26 is fixed to the rear surface of the base plate 24 by adhesives. The radiating plate 26 serves to prevent heat generated by the heating elements 25, from being accumulated in the base plate 24. In the prior-art head of this type, however, the thickness of the radiating plate 26 is only about 5 times that (about 1 mm) of the base plate 24, so the effect of heat radiation is insufficient for the heat generation of the large number of dots as in the case of the picture print. Even when radiating fins are further mounted in contact with the rear surface of the radiating plate 26 or when a forced cooling fan is provided, rise in the temperature of the base plate during the printing operation cannot be satisfactorily suppressed yet. This has led to the problem that a density varies greatly in the print, to degrade the picture quality of the print. Meanwhile, the rear surface of the radiating plate 26 is formed with various mounting holes for the purposes of, e. g., positioning the head and printer mechanisms and imparting the contact pressure of the head. The positional accuracy of the heating element array of the head and the stability as well as the uniformity of the contact pressure between the heating elements and recording paper exert important influences on the picture quality of the print. For this reason, the mounting holes for positioning the head and the holes for mounting springs for compressing the heating elements to the recording paper are usually formed at positions which substantially correspond to the heating element array 27 indicated by a broken line in FIG. 4. By way of example, in FIG. 4, symbols 28a and 28b denote the mounting holes for positioning the head, numerals 29 the holes for mounting the compression springs, numeral 30 a hole for mounting a temperature detecting thermistor 31, and numerals 32 holes for mounting the radiating fins, and among them, the holes 28a and 29 are formed at the positions corresponding to the heating element array 27. In this regard, since the thickness of the radiating plate 26 is not sufficiently great relative to the diameters of the mounting holes in the prior-art head, the mounting holes are inevitably formed into through holes in order to attain screwing strengths. Consequently, parts corresponding to the mounting holes under the base plate become air (heat insulating) parts, so that the heat generated by the heating elements over the mounting holes is difficult to radiate. This has led to the problem that the temperatures of some of the heating elements heighten to incur a density irregularity on the printed picture. When the positions of the mounting holes are widely shifted from the position of the heating element array, the density irregularity ascribable to the radiating property is avoided. However, it becomes difficult to attain the positional accuracy of the heating elements or the accuracy of the contact pressure of the heating elements relative to a platen roller, to bring about the different problem that roughness is prone to appear on the printed picture.
In addition, a method in which printing conditions are controlled on the basis of the detection of the temperature of the head is employed for attaining a high printed picture quality. The prior-art head, however, has had the problem that, since the radiating plate is thin, the temperature distribution thereof is apt to become nonuniform, so the detection of the average temperature of the whole head is difficult.
As described above, the prior-art thermal recording head has involved the problem that the heat radiation for the heat generation by the large number of dots is not satisfactorily considered for the radiating plate, so the density varies greatly in the print, and the problem that, since the radiating plate is formed with the through holes, the radiating property thereof worsens partly in the direction of the heating element array, so the density irregularity appears.